Two-piece cans are the most common type of metal containers used in the beer and beverage industry and also are used for aerosol and food packaging. They are usually formed of aluminum or tin-plated steel. The two-piece can consists of a first cylindrical can body portion having an integral bottom end wall and a second, separately-formed, top end panel portion which, after the can has been filled, is double-seamed thereon to close the open upper end of the container.
An important competitive objective is to reduce the total can weight as much as possible while maintaining its strength and performance in accordance with industry requirements. For pressurized contents such as soft drinks or beer, the end panel must be made of a metal thickness gauge that is on the order of at least twice the thickness of the side wall. Accordingly, to minimize the overall container weight the second end panel should be diametrically as small as possible and yet maintain the structural integrity of the container, the functionality of the end, and also the aesthetically-pleasing appearance of the can.
In most cases, containers used for beer and carbonated beverages have an outside diameter of 2-11/16 inches (referred to as a 211-container) and are reduced to open end diameters of (a) 2-9/16 inches (referred to as a 209-neck) typically in a single-necking operation for a 209 end; or, (b) 2-(7.5)/16 (referred to as a 2071/2-neck) typically in a double-necking operation for a 2071/2 end; or, (c) 2-6/16 (referred to as a 206-neck) in a triple- or quad-necking operation for a 206 end. In the future, it is expected that even smaller diameter ends will be used, e.g., 204, 202, 200 or smaller. Further, different can fillers use cans with varying neck size. Hence, it is very important for the can manufacturer to quickly adapt its necking machines and operations from one neck size to another.
Until recently, the process used to reduce the open end diameter of two-piece containers to accommodate smaller diameter second end panels typically comprised a die necking operation wherein the open end is sequentially formed by one, two, three or four die-sets to produce respectively a single-, double-, triple- or quad-necked construction. Examples of such proposals are disclosed in U.S. Pat. Nos. 3,687,098; 3,812,896; 3,983,729; 3,995,572; 4,070,888; and, 4,519,232. It will be noted in these instances that for each die necking operation, a very pronounced circumferential step or rib is formed. This stepped rib arrangement was not considered commercially satisfactory by various beer and beverage marketers because of the limitations on label space and fill capacity.
In an effort to offset the loss of volume or fill capacity resulting from the stepped rib configuration of the container, efforts have been directed towards eliminating some of the steps or ribs in a container neck Thus, U.S. Pat. No. 4,403,493 discloses a method of necking a container wherein a taper is formed in a first necking operation and this tapered portion is reshaped and enlarged while the angle of the taper is increased. A second step or rib neck is then formed between the end of the tapered portion and the reduced cylindrical neck.
U.S. Pat. No. 4,578,007 also discloses a method of necking a container in a multiple necking operation to produce a plurality of ribs. The necked-in portion is then reformed with an external forming roller to eliminate at least some of the ribs and produce a frustoconical portion having a substantially uniform inwardly curving wall section defining the necked-in portion.
In recent times beer and beverage marketers have preferred a neck construction having a relatively smooth neck shape between, for example, the 206 opening and the 211 diameter can. This smooth can neck construction is made by a spin necking process, and apparatus as shown, for example, in U.S. Pat. Nos. 4,058,998 and 4,512,172.
For various reasons, the can manufacturing industry believed that spin necking was the only method of producing a smooth neck configuration. Applicants have found, however, that presently available spin necking devices and their operation are not entirely satisfactory. It was found that commercial spin necking stretches and thins the neck metal and thereby tends to weaken the neck. From applicants' experience, at commercial production speeds, the presently known spin forming apparatus and process requires frequent maintenance and attention and yet produces considerable scratches and ridges in the neck surface that are undesirable in the marketplace. Moreover, the spin-necked containers did not meet the performance standards set by the equivalent-sized die necked container. For example, applicants experienced distortions in the symmetry of spin-necked containers, crush problems and uneven edges, which resulted in variations in flange width.
While presently-available spin necking equipment and operations have various shortcomings, no one to the knowledge of the Applicants has tried to make high-performance smooth necked cans by die necking, as taught herein. Apparently, the industry believed that the die necking process could not be effective in producing a totally smooth neck construction in a fast, economical, efficient and reliable manner.